I'm not attracted to underage girls, that's disgusting! The only reason I thought she was an adult was because she had breasts and was shaking her ass in a skimpy dancing outfit.
No, it's not.
Age of consent is just an arbitrary number, the fact that countries have different ages for that proves just how arbitrary that is. Being attracted to somebody under an imaginary age line isn't in any way disgusting. If it's a long way under then yeah, that's not right. But just a year or two? Well firstly that would be completely legal in another country and it's entirely possible that somebody looks older or younger than they actually are. If they're attractive then they're attractive. You shouldn't have to ask their age before you can decide if you think they're attractive or not.
I don't think being attracted to an underage girl is disgusting at all. As Ka-pi said, the number is BS. But, aside from that, you can't help who you're attracted to. What matters is what you do with that attraction. If you say, "wow, hot chick", then drive on by and go about your day, there's nothing wrong with that. But, if you see a 13 year old girl that's attractive, then try to fuck her, you have a real problem. I think we can agree that that's disgusting.
But, let's take a beautiful 17 year old. In most places, that's one year from being considered an adult. Are you sick in the head if you fuck her today, and the envy of all of your friends if you want until next year to do it? In some people's opinions, yes. But, as far as I'm concerned, if you find a girl in her late teenage years, that seems relatively mature, is interested, and, most importantly, you don't use your superior manipulation skills to fool her into sleeping with you, you're done nothing morally wrong if you end up in bed with her.
tl;dr - We have to apply some common sense, and evaluate each situation individually to determine what is actually wrong.